When the barometric pressure is reduced to 380 mm Hg (1/2 atmosphere) I found that platelets aggregated independently of pO2. By analogy with the vascular occlusion caused by sickled erythrocytes in sickle cell crisis the hypothesis is proposed that reduced atmospheric pressure (decompression) at high altitude causes platelet aggregation which partially occlude arterioles and causes acute mountain sickness (AMS). In both disabilities the damage is caused by hemostatic plugs adhering to the endothelial wall, plugs composed in the one instance of sickled erythrocytes, in the other of aggregated thrombocytes. Because I have found that piracetam (2-oxo-1 pyrolidine acetamide) at concentration of 0.1 to 0.5 mM prevents thrombocyte aggregation, I suggest that the drug might prevent or alleviate AMS. I also found that piracetam renders fibrin polymer more soluble. I predicted and found that NaBr (40 mM) prevented platelet aggregation. It is interesting to note that Dr. Ricardo Manrique, University of Sao Paulo, Brazil found that in a clinical trial piracetam and pentoxofilin (another anti-platelet drug) prevented AMS.